Many mobile computing and communicating devices rely upon standard battery cells for providing power on which to operate. Though disposable battery cells, such as alkaline cells, are a well-known and reliable technology, it is common in such mobile devices to employ rechargeable battery cells. These rechargeable batteries depend on a number of known cell types, including Ni-Cad, Ni-MH, and Li-Ion cells. All these cells are known to those of skill in the art, as are some of their deficiencies. One of the known deficiencies of the above mentioned rechargeable battery cells is related to the fact that each battery has a finite life span that can be measured in terms of recharge cycles. The process of charging and discharging the cell damages the cell's charge storage capabilities, causing the stored potential, which is typically measured in mA-hours, to decrease over the life of the battery. As the ability to store charge decreases, so does the battery's utility. The life of the battery can be drastically curtailed by improperly charging, or over discharging the battery. Another known deficiency of the above cell types is that the batteries are known to discharge while in storage, though some types of battery are more susceptible to the self-discharge phenomenon than others. As a result of these deficiencies, it is crucial that a user be able to determine the capacity of a battery both prior to and during use.
A state of the art technique for battery capacity reporting relies on the coulomb counter. The principle of operation involved in coulomb counting is computing a coulomb count equal to the coulombs injected into a battery minus the coulombs taken out of the battery. The capacity of the battery is then reported by comparing the coulomb count relative to a reference coulomb count value that corresponds to maximum battery capacity. For instance, if the coulomb count of a battery is half of the reference value, the battery capacity is reported to be 50 percent. Although the coulomb counter addresses battery capacity reporting, it may have several problems. First, the reported capacity may not be meaningful if an accurate reference coulomb count value corresponding to maximum battery capacity is not known. Furthermore, with a coulomb counter it may be difficult to keep an accurate reference coulomb count, particularly when battery capacity decreases over the lifetime of the battery. Further still, with a coulomb counter it may be necessary to know the current battery capacity before beginning the coulomb count.
A limitation of the coulomb counting principle is that it may not be applicable to reporting the capacity of a battery of initially unknown battery capacity: if the capacity of a battery is to be reported using the coulomb count system and method, the battery may have to be taken from it's unknown capacity state to either a fully charged 100 percent battery capacity state or to a fully discharged 0 percent capacity state before the coulomb count can be used. Because the state of the battery is unknown at a certain point, the only way to charge the battery to 100% capacity is to constantly provide charge over an extended length of time. This can result in an overcharging of the cell, which is known to damage to the storage capability of the cell. Conversely, to guarantee that the cell is at 0% capacity, the cell must be completely discharged. It is a known phenomenon that rechargeable batteries are damaged by a full discharge to a complete empty state. Thus forcing a battery to either 100% or 0% capacity will likely damage the cell, which only hastens the time at which the coulomb counting becomes inaccurate.
Further practical limitations exist with coulomb counting techniques. In practice, coulomb counting works by applying an integration over time. The presence of an offset in a coulomb counter may result in the inaccuracy of the coulomb count. This applies even to batteries with an assumed initially known battery capacity, and is compounded with every recharge cycle. This may be especially true if the battery needs to be used for a long period of time between opportunities to reset the coulomb counter. For instance, in a battery that needs to be used for 3 weeks between charges, even small offsets with each charge cycle may accumulate to large inaccuracies in reported capacity.
Other known techniques of battery capacity reporting exist, and are primarily based on measuring battery voltage. The interest in such voltage techniques is due to the technical ease involved in voltage measurement. However, voltage measurement techniques also present the greatest challenges since the relationship between battery voltage and battery capacity is plastic, i.e. for any given battery capacity, the measured battery voltage can vary greatly. The presence of such variations prevent the systematic reporting of meaningful battery capacity values. The variations are small if the current draw is fairly constant over the lifetime of the battery, so there are situations where a direct voltage to capacity mapping will suffice.
Many battery capacity reporting solutions assume a fairly constant current draw for the major mode of operation, and only report capacity in this mode. For example, most cell phones only report battery capacity when they are not charging. Once they start charging, their battery gauges stop indicating battery capacity. However, in applications where a battery is recharged while the system is running, such a change in state from discharging to charging, or vice versa, may break any assumptions about constant current draw.
Batteries have known characteristic charge and discharge curves. FIG. 1 illustrates a charge curve 140 and a discharge 130 curve for a battery. These curves relate battery voltage 120 to percent capacity 110 for a rechargeable battery. The curves provide a model 100 for a battery. In the model, percent battery capacity 110 is related to battery voltage 120 in either a discharging state, shown by discharge curve 130, or the charging state shown by charge curve 140. Illustrated is a multiplicity of points such as point 132 on the discharging curve 130 and of point 142 on the charging curve. Interpolation can be used to provide capacity values 110 for voltages 120 that lie between points for which values are known.
In reference to FIG. 1, the details of a charge state capacity model 100 are described. The relationship between battery voltage 110, battery charge state and capacity 120 is illustrated by two curves 130,140. A first curve 140 corresponds to a positive battery charge current or charging battery charge state, and a second curve 130 corresponds to a negative battery charge current or discharging battery charge state.
Although not expressly shown in the drawings, the charge state capacity model 100 can use more than one pair of curves. Each curve is a function of both the battery charge current and the battery charge state. The charge state is used to select at least one curve from a multiplicity of charge curves. Each curve is a function of the battery charging current, and relates battery voltage to capacity. For example, when the battery is in a first charge state, such as the charging state, a first charge curve corresponding to the charging state is utilised. When the battery is in a second charge state, for instance the discharging state, a second charge curve corresponding to the discharge state is utilised. The charge curves are such that given a battery voltage value and a charge curve, it is possible to obtain a corresponding capacity value from the charge curve.
Though it is possible to determine the capacity of a battery by measuring the voltage of the battery and examining the model, it should be noted that the existence of two distinct curves presents a problem. When a battery is charging and is at 50% capacity, it has a defined voltage level. If the battery charging is terminated when the battery is at 50%, the voltage of the battery does not instantly decrease to the voltage that corresponds to 50% capacity on the discharge curve. Instead the voltage decays to that level over time. The voltage of a 50% battery in a charging state is equivalent to the voltage of a 60-70% battery in the discharging state. As a result, most voltage based battery capacity reporting devices report a capacity jump when charging is ended. Similarly, there is a reported battery capacity drop when charging is started. These abrupt changes in capacity are inaccurate, and cause confusion among users.
There remains a further need for a system and method of battery capacity reporting based on battery voltage that overcomes the limitations present in the plastic relationship between battery voltage and battery capacity.
There remains a further need still for a system and method of battery capacity reporting which systematically reports a meaningful battery capacity value whether the battery is being discharged or charged, and which does so regardless of the presence of transitions between the charging and discharging of the battery.